Field of the Invention
An imaging method is generally used to nondestructively examine objects or substances and is largely classified into two types of an image detection type using a monochromatic light and an image detection type using a spectroscopic method. These types have advantages and disadvantages, but the image detection type using a monochromatic light is much used for a case requiring relatively high intensity such as a transmission image.
Description of the Related Art
In general, the higher the resolution, the lower the DOF (Depth of Focus), so these techniques still have limits.
In particular, an optical system with high resolution has a small DOF, so it is required to scan a focal point in a depth direction of an object in order to examine the internal structure of an object having a predetermined volume in a nondestructive way. This problem takes more time to make 3D CT (Computerized Tomography) on the basis of a projection absorber image, when such depth-directional scanning is not performed, a projection image with very low accuracy is made and the quality of the image is deteriorated.
Further, the smaller the focal distance, the higher the detection resolution, and the distance between an object and a lens should be small to improve the resolution. Accordingly, a working distance is considerably limited.
These problems can be overcome using a Bessel beam known as a non-diffracting beam proposed by Durnin and the Bessel beam has been known are being able to increase the DOF and improve resolution (available under a wavelength) in an optical system.
On the other hand, for acquiring an image in a focal plane in real time, a method of acquiring an image by directly using a focal plane array detector or a method of acquiring an image by combining a linear array detector or a single point detector with a scanning unit has been known.
In particular, among these methods, a method that can achieve highly sensitive detection at a low cost with high radiation intensity per unit area is a method that focuses an electromagnetic wave to one point and detects an electromagnetic wave reflecting from or passing through a specimen while changing the travel direction of the electromagnetic wave with a single detector, using high-speed raster scanning (polygon mirror, galvano mirror, and the like) at a high speed, in order to efficiently use the energy of an incident electromagnetic wave.
However, most electromagnetic beams used in the high-speed raster scanning are focused in the type of a Gaussian beam, so there are many limits in improvement of resolution and DOF, as described above.